Play the ones that don't have you permenatly lose units when they die, that is the easy mode.

I dunno all the games that have this feature but i think the Awakening and Fates games are the only two as fas as i know. I honestly don't play this game as hard core fan does, back in my day we had SHINING FORCE Back when the creators wasn't making Mario tennis and golf games... or golden sun. There was shining force and Tactics ogre and Final fantasy tactics.

You could try shadow dragon on the ds, its technically the first game of the series however it has some different mechanics than recent entries. For recent titles play awakening its fun and challenging and you dont really need any prior knowledge of the game. I would also recommend fates but i believe it is a bit more difficult on beginners, well at least conquest is.

If it's something to play as a one off, then Awakening and Fates are very good.
If this is a series you're interested in properly getting into, then I'd recommend starting around 7. This way you'd appreciate the new changes each game brings, rather than feeling like each game you play has less and less by going in reverse order.